Ventilation / heat recovery / energy efficiency
This is a thought process about ventilation and energy efficiency, in ten steps:
1. According to ASHRAE 62.2, a house with 1,500 SF occupied area and two people needs 30 CFM of ventilation air (7.5 CFM per person plus 1 CFM per 100 SF). That’s 1,800 CF of fresh air per hour and 43,200 CF per day. Logically there may be nobody home sometimes and more than two people other times, but let’s consider this the average ventilation volume needed each day for this small household.
2. For optimal energy efficiency, ventilation air volume should not exceed the required 43,200 CF per day. There’s a penalty in space conditioning energy when more outdoor air comes indoors, bypassing the thermal enclosure.
3. Let’s assume that ventilation air volume can be somewhat sporadic (rather than a steady, continuous 30 CFM) without ill effects. This becomes helpful because spot ventilation (on demand) will exceed 30 CFM for a limited time, for example a bath fan running at 50-70 CFM. To avoid over-ventilating, spot ventilation will contribute toward the overall ventilation air volume.
4. Let’s imagine a theoretically optimal exhaust-only setup: bath fans with a combination of demand use and programmed fan cycling that totals close to the ideal 43,200 CF per day. (More on this later.) For now let’s ignore the kitchen range vent. There’s a condensing clothes dryer and no other exhaust devices.
5. A theoretically perfect HRV’s heat recovery core would reduce the delta-T between incoming and outgoing air to zero. If outdoor air is 35°F and indoor is 65°F, delta T is 30°F. Air passing through the core would depart at 50°F both ways. Therefore the perfect HRV saves half the energy lost to ventilation air.
6. The Venmar EKO 1.5, an energy efficient HRV, operates at about 60 CFM at the lowest setting, or 86,400 CF per day. In continuous use this would provide enough ventilation for six people in a house this size.
7. With 60 CFM split among four exhaust registers, each will move 15 CFM. That’s not much for demand exhaust in a bathroom.
8. There could be controls provided in the bathrooms to boost the Venmar to maximum air volume on demand, about 120 CFM. That’s 30 CFM for the bathroom where the boost gets triggered, and the same for the other 3 exhaust locations. If the HRV runs on boost for one hour per day, that adds 3,600 CF to the total ventilation volume (now 90,000 CF).
9. Mechanical ventilation equipment draws electric power. This is another factor in energy efficiency because some devices use considerably more power than others. An efficient exhaust fan (e.g. WhisperGreen series) moves ~10 CFM per watt, whereas the efficient Venmar HRV moves ~2.5 CFM per watt.
10. Even a perfect HRV would break even on space heating energy if it moved double the required air volume, based on point #5. In this example, the Venmar moves double the air volume and uses 5x the electric power per unit of air volume. That is, the HRV uses 10x as much electricity as the exhaust only system in operation.
Now, unless I’ve missed something important, the HRV seems unlikely to come out ahead on energy efficiency. However, it would be possible to program it to run on demand at high speed and remain off much of the time, just like the exhaust only system does here. I would guess the reason the Venmar’s lowest speed is 60 CFM rather than lower (say 30 CFM) is to make it more usable as a bath fan replacement. But even on high speed it’s somewhat ineffective at spot ventilation with 30 CFM per vent (assuming four exhaust locations).
Here’s what I have in mind for an exhaust only system: In each of three bathrooms there’s a fan set to 60 CFM with a motion sensor and 60-minute delay timer. Three fans need to run for 12 hours per day (combined runtime) to meet the air volume target. Using a kitchen range vent (100 CFM on low setting) would reduce this target. At busy times of the day all three bath fans might run at once (moving 180 CFM) while at quiet times they remain still. Ventilation responds to activity, there’s no need to switch fans on/off and they should stay mostly imperceptible (0.3 sones).
The plan for makeup air is a single 6” diameter duct (like an HRV intake) that branches to four supply registers. The intake passes through a filter and a robust gate valve. This valve can be shut for the purposes of blower door testing, which may or may not be cheating. Exhaust ducts can employ redundant backflow dampers (one near the outside of the wall, one near the inside) for the same purpose.
Comments are welcome.
Posted Mar 30, 2011 11:13 PM ET
Other Questions in Green building techniques